Akt attenuation of the serine protease activity of HtrA2/Omi through phosphorylation of serine 212

The serine protease HtrA2/Omi is released from the mitochondria into the cytosol following apoptosis stimuli, leading to the programmed cell death in caspase-dependent and -independent manners. The function of HtrA2/Omi closely relates to its protease activity, which is required for cleavage of its substrate such as the members of the X-linked inhibitor of apoptotic protein family. However, the regulation of HtrA2/Omi by signaling molecule has not been documented. Here we report that serine/threonine kinases Akt1 and Akt2 phosphorylate mitochondria-released HtrA2/Omi on serine 212 in vivo and in vitro, which results in attenuation of its serine protease activity and pro-apoptotic function. Abolishing HtrA2/Omi phosphorylation by Akt through mutation of serine 212 to alanine (HtrA2/Omi-S212A) retains its serine protease activity and induces more apoptosis as compared with wild-type HtrA2/Omi. Conversely, HtrA2/Omi-S212D, a mutant mimicking phosphorylation, lost the protease activity and failed to induce the programmed cell death. Furthermore, the phosphorylated HtrA2/Omi fails to cleave X-linked inhibitor of apoptotic protein without interfering with their complex formation. In addition, Akt inhibits the release of HtrA2/Omi from the mitochondria into the cytoplasm in response to cisplatin treatment. These data reveal for the first time that HtrA2/Omi is directly regulated by Akt and provide a mechanism by which Akt induces cell survival at post-mitochondrial level.